In optical recording, increasingly the trend is towards miniaturisation of both the optical record carriers and the devices used to scan (e.g. write to and/or read from) the carriers. Examples of optical record carriers include CDs (compact discs) and DVDs (digital versatile discs).
In order for the optical record carriers to be made smaller, without a decrease in information storage capacity, the information density on the carrier must be increased. Such an increase in information density must be accompanied by a smaller radiation spot for scanning the information. Such a smaller spot can be realised by increasing the numerical aperture (NA) of the objective system used for focusing a radiation beam in the scanning device on the record carrier. Consequently, it is desirable to have a lens with a high numerical aperture (e.g. NA=0.85).
Conventional high NA objective lenses consist of two elements in order to ease the manufacturing tolerances, at the expense of introducing an extra assembly step to align the elements making up the objective lens.
The Japanese article “Single Objective Lens Having Numerical Aperture 0.85 for a High Density Optical Disk System” by M Itonga, F Ito, K Matsuzaki, S Chaen, K Oishi, T Ueno and A Nishizawa, Jpn. J. Appl. Phys. Vol. 41. (2002) pp. 1798–1803 Part 1, No. 3B March 2002, describes a single objective lens, having two aspherical surfaces, with a relatively high NA of 0.85. The lens is made of glass. The lens diameter is 4.5 mm, and the lens has an aperture diameter of 3.886 mm. This single element lens does not require the extra alignment assembly step needed by the two-element objective lens. Because of the high value of NA, the objective lens becomes more susceptible to variations in the manufacturing process i.e. manufacturing tolerances. Therefore, for these high NA objective lenses the manufacturing tolerances play an even more important role in the designing process than was the case for objective lenses having a lower numerical aperture.
In order for scanning devices to decrease in size, it is desirable that the components within the scanning devices (such as the objective lens) are made as small as possible.
However, it is not possible to simply scale down large lens designs to produce smaller lenses, as the lens design is dependent upon the properties of the optical recording medium. For instance, the lens design is dependent upon the properties of the transparent layer that typically covers the information layer on an optical record carrier, and which the scanning radiation beam must traverse. In the scaling down process the thickness of the cover layer of the disc remains unaffected (the same record carrier is likely to be used for both the normal sized objective lens and the small sized objective lens). Hence, the design of a small sized objective lens suitable for scanning the optical record medium will be substantially different from the design of a normal sized objective lens.
Further, whilst it is desirable that the objective lens is formed of a single element (assembling two small elements is difficult and therefore rather expensive), forming a single element solely out of glass is relatively expensive. The glass moulding production process requires high temperatures to melt the glass, and relatively large forces to shape the melted glass, thus making the resulting lens a relatively expensive component.
A cheaper alternative method of manufacturing a single element lens is to form a synthetic resin on a flat or spherical substrate (such as glass). For instance, glass spheres are relatively cheap to manufacture, and so truncated glass spheres are ideal substrates. Synthetic resins may be applied to the surface of the substrate so as to provide the desired (e.g. aspherical) surface shape. U.S. Pat. No. 4,623,496 describes how such a liquid synthetic resin can be applied to a substrate, with the synthetic resin being subsequently cured so as to form a layer having a predetermined desired aspherical curved characteristic.
It will be appreciated that design constraints for lenses formed using a synthetic resin on a substrate will differ from design constraints for lens formed from a single substance such as glass. For instance, the synthetic resin will typically have a different refractive index than the substrate.
It will also be appreciated that as lenses are made smaller, high NA lenses remain susceptible to variations in the manufacturing process i.e. manufacturing tolerances.
FIG. 1A shows an example of an objective lens 18, having a glass body 200 with a substantially spherical surface 181, and a substantially flat surface 182. Such a glass body would subsequently have at least one layer of a synthetic resin applied to the first surface 181 so as to form an aspherical surface. It will be appreciated that if the glass body is formed or aligned incorrectly, then the performance of the lens formed with the addition of the resin will be impacted. The lens is of total thickness t along the optical axis (i.e. thickness of body plus resin layer(s)).
In the examples shown in FIGS. 1A–1D, two separate layers of resin 100, 102 are applied to respective surfaces 181, 182 of the glass body 200. Each layer of resin 181, 182 is shaped so as to form a respective aspherical surface.
Subsequent FIGS. 1B, 1C and 1D respectively illustrate how the substrate shape and orientation can vary due to variations in thickness, decentre and tilt of the two aspherical surfaces relative to the desired optical axis 19 (in each instance, the original position of the surface 181 is illustrated by a dotted line).
FIG. 1B illustrates the overall thickness of the lens being greater than the desired thickness, in this instance due to the spacing in between the surfaces 181, 182 being larger than desired. However, it will be appreciated that the two surfaces could in fact be spaced closer together than desired as well.
FIG. 1C illustrates decentre of the two aspherical surfaces. In this example, the glass body 200 has been located shifted in a direction perpendicular to the ideal position relative to the desired optical axis 19, with the centre of the aspherical surface 100 being off the desired optical axis 19, whilst the aspherical surface 102 remains centred on the axis 19.
FIG. 1D illustrates how the glass body, including surface 181, is tilted i.e. rotated in relation to the desired rotationally symmetric position along the principal axis, resulting in a tilt of the upper aspherical surface 100, relative to the lower aspherical surface 102.
It is an aim of embodiments of the present invention to provide an objective lens formed from a synthetic resin on a substrate material capable of withstanding reasonable manufacturing tolerances.
In optical scanning devices, radiation beams may enter the objective lens obliquely, due to inaccurate alignment of the objective lens within the scanning device, variations in the position of the recording carrier relative to the scanning device, or due to radiation beams being utilised that do not travel along the optical axis. For instance, such off-axis beams are typically used to provide information on positioning of the scanning radiation spot on the record carrier.
Such oblique beam entrance results in wave-front aberrations. Typically an allowance in the root mean square of the optical path difference (OPDrms) of approximately 0.07λ (where λ is the wavelength of the relevant radiation beam), in total is allowed for wave-front aberrations of the scanning beam for the total optical scanning device, such that the system is diffraction limited. It can be convenient to express the OPDrms in mλ (where 0.001λ=1 mλ). The field of the lens system is the area within which oblique beams generate an OPDrms of less than 15 mλ. The field of view of the lens system is twice the field.
It is an aim of embodiments of the present invention to provide an small sized high NA objective lens formed of a synthetic resin on a substrate that is tolerant to oblique beam entrance to the lens and tolerant for manufacturing errors.